The task of diffraction limited light focusing inside transparent or semi-transparent media is important in techniques presuming a use of high numerical aperture (NA) objectives, for example, various types of microscopy including confocal microscopy (S. Hell et al., “Aberrations in confocal fluorescence microscopy induced by mismatches in refractive index,” Journal of Microscopy 169(3), 1993), multi-focus microscopy (S. Abrahamsson et al., “MultiFocus Polarization Microscope (MF-PolScope) for 3D polarization imaging of up to 25 focal planes simultaneously,” Opt. Express 23, 2015).
Other applications examples are cutting or drilling of glass, sapphire and other brittle materials using ultrafast lasers, laser-induced refractive index variation of glass (N. Huot et al., “Analysis of the effects of spherical aberration on ultrafast laser-induced refractive index variation in glass,” Opt. Express 15, 2007), nanostructuring in glass for optical data storage (J. Zhang et al., “Seemingly Unlimited Lifetime Data Storage in Nanostructured Glass,” Phys Rev Lett 112(3), 2014, and E. Glezer et al., “Three-dimensional optical storage inside transparent materials,” Opt Lett 21, 1996), polarization converters (M. Beresna et al., “Radially polarized optical vortex converter created by femtosecond laser nanostructuring of glass,” Appl Phys Lett, 98, 2011), holographic and other optical data storage techniques.
All of these microscopy, optical data storage and material processing techniques imply light beam focusing in transparent media in small spots, with the size of few micrometers being close to a wavelength value. A common feature of these techniques is using high-NA objectives which aberration correction is provided for a particular working plane in air or inside a transparent medium, for example on back surface of a cover slide; by diffraction limited focusing a spot size in the pre-determined working plane is defined by wave nature of light, i.e. by diffraction limitation.
When focusing at other than pre-determined depths inside the bulk medium there appears longitudinal spherical aberration, and resulting spot size is defined rather by this geometrical aberration—for numerical aperture more than NA0.4 the resulting focused spot can become several times larger than the diffraction limited one.
FIG. 1 illustrates appearing of spherical aberration by focusing the light inside a transparent medium, for example glass, having a flat boundary surface separating the said medium and air. Beam of light 1 propagating from air into the transparent medium 4 is focused in virtual point F′ being located at depth s0 inside the transparent medium. Paraxial focus F″ of the beam after refraction on the boundary flat surface locates at depth s0′ from that surface. Refraction of a ray on optical surface obeys the well-known Snell's law (Smith, W. J., Modern Optical Engineering, McGraw-Hill, New York, 2000), as result different rays of the beam after refraction intersect optical axis in different points, and the bigger is a ray slope angle the bigger is distance between the paraxial focus F″ and the point of optical axis intersection, that distance is called as longitudinal spherical aberration.
For rays shown in FIG. 1, slope angles wI<wII, correspondingly distances to intersection points are sI′<sII′, and longitudinal aberration ΔsI′ for the ray with slope angle wI is smaller than ΔsII′ for the ray with slope angle wII. The longitudinal aberration is positive when light beam propagating from air into glass, i.e. into the medium which refractive index is higher than one of the air. The higher is optics NA or deeper is focusing inside the transparent media, the bigger is longitudinal spherical aberration and stronger scattering of light energy.
Aberration has, inevitably, negative influence on the concentration of laser energy in laser processing techniques and reduces physical resolution, contrast and image intensity in optical microscopy (S. Hell et al., “Aberrations in confocal fluorescence microscopy induced by mismatches in refractive index,” Journal of Microscopy 169(3), 1993, and R. Simmonds et al., “Three dimensional laser microfabrication in diamond using a dual adaptive optics system,” Optics Express 19(24), 2011) and reduces density of data recording in media in data storage techniques. There appears also an aberration induced shift of effective focus from a nominal focus position (S. Hell et al., “Aberrations in confocal fluorescence microscopy induced by mismatches in refractive index,” Journal of Microscopy 169(3), 1993)—this is very important in confocal microscopy and other measurement techniques.
Several technical solutions are known to overcome the negative effects from aberration induced by internal focusing in a transparent media.
The German Patent Application DE102010039746A1 to Glasenapp describes a method of using a correcting optical element comprising freeform surfaces and locating before a microscope objective; the correcting optical element introduces in a control manner aberration that compensates the aberration induced by internal focusing in transparent media. Obviously, design of that correcting optical element should be adapted to a particular microscope objective with taking into account features of its optical design; this narrows the range of applicable objectives. An important disadvantage of this method is the use of freeform optical surfaces in which manufacturing is expensive.
Another approach to building objectives optical systems with movable lens groups is described in U.S. Pat. No. 5,940,220 to Suenaga et al. and U.S. Pat. No. 8,705,178 to Fujita. U.S. Pat. No. 6,563,634 to Shimada et al. and U.S. Pat. No. 7,903,528 to Kimura et al. describe solutions with motorized motion of movable lens groups. U.S. Pat. Nos. 8,659,827 and 9,195,041 both to Redford, U.S. Pat. No. 6,721,259 to Yamamoto et al., U.S. Pat. Nos. 7,177,101 and 7,602,561 both to Tanaka et al. present microscope optical systems where aberration compensation is achieved by moving lens groups not in the objective but in other parts of the imaging optical systems.
The approach of using movable lens groups is widely used in imaging using high NA microscope objectives and provides good performance and stable results when operating in particular working planes of a pre-determined range of focusing depths. However technical realizations are rather complicated, provide small range of depths with aberration-free focusing and don't allow compensating the spherical aberration simultaneously in several working planes separated along optical axis required in multi-focus microscopy.
Aberration correction is one of main application tasks for adaptive optics. Optical systems with Spatial Light Modulators (SLM) are described in U.S. Pat. No. 7,065,013 to Yasuda et al., U.S. Pat. No. 7,903,528 to Kimura et al., U.S. Pat. No. 8,526,091 to Ito et al., U.S. Pat. No. 8,659,824 to Kato et al., and paper (H. Itoh et al., “Spherical aberration correction suitable for a wavefront controller,” Optics Express 17(16), 2009). Various technical solutions based on deformable mirrors are presented in U.S. Pat. No. 8,629,413 to Betzig et al., U.S. Patent Application No. 2015/0185474 to Goldberg et al. and papers (M. Booth et al., “Adaptive aberration correction in a confocal microscope,” PNAS 99(9), 2002 and R. Simmonds et al., “Three dimensional laser microfabrication in diamond using a dual adaptive optics system,” Optics Express 19(24), 2011).
Common disadvantages of the technical solutions based on adaptive optics are complexity of realization leading to high costs and low reliability. These methods provide aberration correction for a particular working plane and can't be used for simultaneous correction of aberration, induced by internal focusing, in several working planes that is required in multi-focus microscopy.
U.S. Pat. No. 6,064,529 to McDonald et al. describes the focusing optical system composed from a movable objective and a fixed corrector lens co-operating to compensate negative spherical aberration occurring in the media of optical data storage disks. As discussed above, the longitudinal spherical aberration induced by focusing inside transparent media is positive. Therefore, there are limitations of using this technical solution in tasks of light focusing inside transparent or semi-transparent media with high NA. This method provides aberration correction for a particular working plane and can't be used for simultaneous correction of aberration, induced by high NA internal focusing, in several working planes that is required in multi-focus microscopy.
From the point of view of modern requirements to light focusing in laser material processing and various implementations of optical data storage and microscopy the conventional techniques are not optimal. As such, there is needed an efficient affordable apparatus and system capable to provide light focusing inside transparent or semi-transparent media characterized by numerical aperture up to NA1.3, aberration correction corresponding to diffraction limited focusing, and fulfilling these focusing conditions simultaneously in a single or multiple working planes separated along optical axis.